1. Field of the Invention
The present invention relates to a load driver circuit suitable for an ignition device in an internal combustion engine. More specifically, the present invention relates to a load driver circuit having a control circuit that controls the on/off cycling of a semiconductor switch element, and a protection circuit for protecting the control circuit from a surge current.
2. Description of Related Art
Electric parts that are mounted in an automobile are required to provide both reliability and resistance against severe environments and accompanying electrical anomalies such as surge currents. Up to now, in order to address anomalies such as surge currents, the control circuit and the protection circuit are disposed on the same substrate in the ignition device for an internal combustion engine.
For example, JP-A 2004-335979 describes a semiconductor substrate having a control circuit and a protection circuit. The protection circuit includes plural protection elements having the same wiring length, the same wiring width, and the same wiring resistance. The protection elements are connected in parallel to each other to form the protection circuit. The protection circuit also includes an npn transistor whose base is open.
FIG. 8 is a circuit diagram showing a conventional protection circuit 100 formed of plural npn transistors 101, the bases of which are open. The plural transistors form a reverse transistor system in which the emitter sides have a relatively higher potential, the collector sides have a relatively lower potential, and the bases are open. In the above circuit configuration, when a voltage across the npn transistor 101 in an opposite direction such as between the emitter and the base, is greater than or equal to a withstand voltage, for example, 24V, due to breakdown of the npn transistor 101, the surge current escapes to GND.
However, since the bases of the npn surge protection transistors 101 are open, the base potential can become unstable. For example, when a potential is applied to the bases in the form of noise, npn transistors 101 can be turned on in parasitic operation resulting in a risk of malfunction.
Also, because of the open base configuration, the current amplification factor (Hfe) must be reduced, for example, by lowering the impurity concentration of an emitter layer as compared with the impurity concentration of a base diffusion layer. However such lowering of the impurity concentration can cause the dynamic resistance to become high, thereby making it difficult for the npn transistor 101 to absorb the surge current when needed.